Patent GB-1476983-A (GB)
Title: System for controlling the hydraulic power lift of an  agricultural tractor
IPC: A01B63/111
Filed: 1974-06-10
Abstract: 1476983 Hydraulic control for tractor power lift VALMET OY 10 June 1974 [19 July 1973] 25603/74 Heading A1P A hydraulic control system for a tractor power lift which ensures a gradual and inexcessive response at the beginning and end of each power lift arm movement comprises in serial connection: a hydraulic fluid, i.e. oil, reservoir 7; a pump 1; a control valve assembly 3, which determines whether the power lift arms 9 are raised, lowered or held; and a throttled spill valve assembly 4, which determines the amount of oil flowing, from the the control valve assembly to the power lift cylinder 15. A spill valve assembly throttle rod 12 ensures a gradual response at each end of arm movement and a descent speed regulating throttle 33 in control valve assembly 3 controls the maximum rate of descent. The system further includes an automatic draught control link 30, a manual position control lever 8, a manual pressure control lever 36 and a cam 35 movable with the power lift rock shaft, all of which are linked to a linkage 11, 41 that controls the position of the throttle rod 12. Control valve assembly 3 This comprises (a) valve 37 whose condition determines whether the oil is returned to the reservoir 7 via line 10 or passed to the lift cylinder 15 via the spillvalve assembly 4, (b) a mechanically operated control valve 5 which is operated by linkage 11, 41 to determine the position of valve 37, (c) a descent valve 31 which, like valve 37 has a control valve 32 mechanically operated by linkage 11, 41, (d) operating pressure control valves 38, 39 which are spring biassed to an extent determined by pressure control lever 36 to govern the operating pressure in cylinder 15, and (e) descent control throttle 33. Spill valve assembly 4 This comprises throttle rod 12 which determines the differential pressure on spill valve 26 and thus the amount of oil flowing back to the reservoir 7 via line 25, and on to the lift via lines 14, 21. During initial and final stages of movement of link element 11 the throttle rod 12 is positioned to cause most of the oil to be spilled. Pressure relief valves 27, 28 are included. Operation To raise the lift arms 9 the manual position control lever 8 is moved to cause linkage 11, 41 to close valves 5, 32. Valves 37, 31 thus close due to increased internal pressure and oil is passed to the spill valve 26 which opens to spill the oil. As the throttle rod 12 is moved by the linkage 11, 41 oil is directed away from the spill valve 26, which therefore closes, towards line 14 and thence cylinder 15. At the end of linkage movement the reverse occurs. Ultimately the linkage opens valve 5 and valve 37 opens to by-pass oil. The pressure within the cylinder is held by closed valves 31, 32. Under the desired operating pressure valves 37, 38, 5 are open and valves 31, 32, 39 are closed. Should the cylinder pressure rise, valve 39 opens to spill oil and cause valve 31 to open until the pressure is decreased. Should the cylinder pressure fall valves 38, 37 close until the pressure rises again. Any movement of the power lift arms causes cam 35 to move the linkage to adjust the valves 5, 32 accordingly.